A polishing apparatus having a polishing tool, such as a polishing tape or a whetstone, is used as an apparatus for polishing a peripheral portion of a substrate, such as a wafer. FIG. 39 is a schematic view of a polishing apparatus of this type. As shown in FIG. 39, the polishing apparatus includes a substrate stage 110 for holding a central portion of a wafer W by vacuum suction and rotating the wafer W, and a polishing head 105 for pressing a polishing tool 100 against a peripheral portion of the wafer W. The wafer W is rotated together with the substrate stage 110 while the polishing head 105 presses the polishing tool 100, whose lower surface (polishing surface) is parallel to a surface of the wafer W, against a peripheral portion of the wafer W, thereby polishing the peripheral portion of the wafer W. A polishing tape or a whetstone may be used as the polishing tool 100.
As shown in FIG. 40, a width of a portion, to be polished by the polishing tool 100, of the wafer W (hereinafter referred to as a polishing width) is determined by a relative position of the polishing tool 100 with respect to the wafer W. The polishing width is typically a few millimeters from an outermost perimeter of the wafer W. In order to polish a peripheral portion of the wafer W with a constant polishing width, it is necessary to align the center of the wafer W with the central axis of the substrate stage 110. Therefore, before the wafer W is placed on the substrate stage 110, centering of the wafer W is performed by holding the wafer W with centering hands 115 as shown in FIG. 41. The centering hands 115 approach the wafer W, which has been transported by a transport robot (not shown), from both sides of the wafer W until the centering hands 115 come into contact with edge portion of the wafer W, thus holding the wafer W. The relative position between the centering hands 115 and the substrate stage 110 is fixed such that the center of the wafer W, held by the centering hands 115, is located on the central axis of the substrate stage 110.
Such a conventional centering mechanism, however, has a limitation in an accuracy of wafer centering. This may result in unstable polishing width. Further, it is possible that the accuracy of wafer centering may decrease due to wear of the centering hands 115. In addition, when the polishing tool 100 is pressed against the peripheral portion of the wafer W, the entirety of the wafer W may warp and a defect may occur in the peripheral portion of the wafer W. In order to prevent such warp of the wafer W, a support stage (not shown), which is provided separately from the substrate stage 110, may be used to support a peripheral area of the lower surface of the wafer W. However, the wafer W will warp if a substrate supporting surface of the substrate stage 110 and a substrate supporting surface of the support stage do not lie in the same plane.